Why Do Priests Bless Water Before Using It?

In short, priests bless water when it’s to be used for a sacred purpose, such as at Mass in preparing the gifts at the altar for the Liturgy of the Eucharist, or in blessing the faithful. In addition, priests bless water for use in holy water fonts, so that the faithful can bless themselves with holy water.

For more on holy water, see this response to a related question and this one as well.